1. Field of the Invention
The present invention relates to a method for scrubbing an amine type extractant after stripping, in more detail, the present invention relates to a method for scrubbing an amine type extractant after stripping which can remove an iron and a zinc efficiently and also regenerate extracting capability of the amine type extractant so that the amine type extractant can be repeatedly reused as it is in the extraction stage in the solvent extraction process, when an amine type extractant is regenerated by scrubbing an amine type extractant containing a ferric (III) chloro complex ion and a zinc chloro complex ion obtained by back extracting a cobalt with a hydrochloric acid aqueous solution from the amine type extractant after extracting cobalt, in the solvent extraction in which cobalt is extracted from a hydrochloric acid aqueous solution containing a nickel and a cobalt and separate from nickel.
2. Description of the Prior Art
Since some amount of cobalt often exists together in raw mineral ores for nickel smelting, cobalt as well as nickel is commonly recovered in the nickel smelting. As a separation method of nickel and cobalt, a method had been used in which cobalt precipitate is separated by using an oxidation neutralization method in which an aqueous solution containing a nickel and a cobalt is neutralized while oxidizing by utilizing a slight difference in susceptibility to oxidation between nickel and cobalt. However, since the above-described oxidation neutralization method was accompanied by coprecipitation of a considerable amount of nickel in the cobalt precipitates formed if cobalt is intended to be sufficiently separated, there was a problem in production efficiency that a reprocessing step for once separated cobalt precipitates is additionally required. In particular, when cobalt content in nickel aqueous solution was high, the oxidation neutralization method could not be a desirable method.
Consequently, in recent years, a solvent extraction has been widely used in which nickel and cobalt are separated by utilizing an extractant consisting of organic solvent. The solvent extraction makes it possible to separate nickel and cobalt contained in the mother liquid for extraction by comparatively simple facilities, and has great effects on cost saving and improved quality. Here, the extractant to be used for separation of nickel and cobalt includes, for example, a phosphate ester type acidic extractant represented by CYANEX 272 (product name), active component being bis(2,4,4-trimethylpentyl)phosphinic acid, produced by Cytec Ind. Inc., or an amine type extractant represented by TNOA.
It should be noted that, when raw material aqueous solution (mother liquid for extraction) for solvent extraction is an aqueous solution of chlorides, an amine type extractant is more commonly used than phosphate ester type one considering separation performance between cobalt and nickel, possibility of crud formation, cost, and the like. In this case, the solvent extraction process comprises an extraction stage in which chloro complex ion of a target metal contained in the raw material aqueous solution is supported by an amine type extractant constituting an organic phase, a stripping stage following thereto in which the target metal supported by the organic phase is eliminated by an aqueous solution forming an aqueous phase, and then a scrubbing stage to separate and remove the components remaining in the organic phase from the organic phase by elimination or substitution reaction with the aqueous solution forming the aqueous phase.
Incidentally, in the solvent extraction operation, an amine type extractant has the following characteristics.
Since the amine type extractant tends generally to have a sufficient extracting character by adding a hydrochloric acid, according to the following chemical equation 1, and extraction of chloro complex ion is performed according to the following chemical equation 2, the amine type extractant exhibits a superior separation characteristic of cobalt and nickel.R3N:+HCl═R3N:H+Cl−
By this reaction, the amine (R3N:) generates an amine added with hydrochloric acid by hydrochloric acid.R3N:H+Cl−+MCl3−═R3N:H+MCl3−+Cl−(wherein, M represents a metal species to form a chloro complex ion of Zn, Fe, Cu, Co, or the like.)
According to this reaction, metal species to form a chloro complex ion of Zn, Fe, Cu, Co, or the like is extracted to form an amine supporting a metal chloro complex ion. It should be noted that, since nickel does not form a chloro complex ion, it remains in the residual liquid for extraction and is thus separated. Accordingly, when nickel aqueous solution contains metals which have higher chloro-complex-forming abilities than cobalt, that is, which are supported more strongly than cobalt, such as iron, zinc, these metals are preferentially extracted and accumulated in the extractant, resulting in such a problem that amount of cobalt to be extracted is reduced.
For this reason, as a regeneration treatment to recover extracting capability, removal of the accumulated impurities, for example, by washing the extractant after stripping with water had been carried out. In the above-described water washing, the extractant is diluted by adding a water and mixing until chloride concentration reaches a level where the impurity element in the extractant cannot form a chloro complex ion, to eliminate the impurity element from the extractant and regenerate extracting capability of the extractant.
However, in order to dilute up to a level where chloro complex ion thereof cannot be formed, for example, in the case of iron, its concentration as chloride in the extractant has to be reduced to around 1 g/L, and a large amount of water is required. Consequently, this method is not industrially advantageous considering water balance and load in waste water treatment process.
Therefore, as an alternative method, a method has been proposed, in which in the scrubbing stage, the extractant is neutralized by contacting with a strong alkaline aqueous solution to remove chloro complex ion accumulated in the extractant. It should be noted that, in such a method using alkali neutralization, the metal chloro complex ion is generally removed as a hydroxide by contacting the extractant with an aqueous solution of sodium hydroxide and the like according to the following chemical equation 3.R3N:H+ MCl3−+3NaOH═R3N:+3NaCl+H2O+M(OH)2 (wherein, M represents a metal species to form a chloro complex ion of Zn, Fe, or the like.)
In this case, iron is removed from the extractant as hydroxide precipitates, and zinc is similarly removed as hydroxide precipitates or hydroxo complex ion. However, the hydroxide often forms precipitates called as crud which tends to cause blockage of equipment piping making the reuse of the extractant difficult. For this reason, a step in which the extractant and the crud were filtered and separated from each other was required to be installed after washing, but the step required cost for the facility and labor because the crud was difficult to filter. In addition, since sodium hydroxide is expensive as an operational material, it was undesirable to use sodium hydroxide for removals of iron and zinc which have only small merit for recoveries thereof. Furthermore, since the amine type extractant itself returns to free amine by the alkali neutralization mentioned above, there was a problem that addition of hydrochloric acid or sulfuric acid had to be done thereafter in order to fully regenerate extracting capability thereof.
As a solution for this problem, for example, a method for regenerating an extractant has been proposed. In this method, in a solvent extraction step for a metal using a water-insoluble tertiary amine extractant solution as an extractant, said extractant accumulated with component having a strong bonding force to the tertiary amine is contacted with a potassium oxalate aqueous solution in the presence of potassium chloride, to regenerate said extracting solvent by back extracting the component having a strong bonding force to the tertiary amine in said extraction solvent, to regenerate by separating and precipitating as hydroxide said component having a strong bonding force to the tertiary amine with adding potassium hydroxide to said liquid after stripping, and said regenerated back extracted liquid is circulated to the regeneration step for extracting solvent (see, for example, Patent Literature 1). By using this method, extracting capability of the extractant can be regenerated by separating the impurity elements such as iron accumulated in the amine type extractant.
However, since potassium oxalate to be used in this method is costly as well as a deleterious substance, there is concerns such as increased load in waste water treatment step, increased risk factors in environment and safety, in its use in practical operation. In addition, there was such a problem that, when an impurity was precipitated in a form of hydroxide by adding potassium hydroxide, operation could become difficult because load of filtration facility was increased by minute precipitates formed or a part of precipitates remained in the extractant as crud.
Further, as another alternative method, a method for regenerating a metal supporting ability of amine had been proposed. This method comprises contacting a liquid organic phase containing a metal supported by amine as a chloride complex and a complexing amine with an aqueous solution containing sulfate ion, nitrate ion and/or phosphate ion but no chloride ion to strip out the metal, and contacting the stripped organic phase with a solution containing a chloride ion to add chlorine to the amine (see, for example, Patent Literature 2). Such regeneration method, in which an amine type extractant is washed by contacting with an acid having a high ion strength such as sulfuric acid other than chlorides, then contacting with hydrochloric acid solution to add chlorine to the extractant, has a feature that the method can regenerate extracting capability by separating the impurity elements without forming the precipitates in the extractant.
However, in order to remove impurity element of chloride complex present in the extractant, it is believed that concentration of chloride in the extractant is required to be reduced to such a low concentration as 1 g/L or less. Dilution of the extractant by mixing a large volume of solution containing an acid to reduce concentration of chloride increases cost of acid to be used. In addition, since a large amount of waste water is generated, processing of the waste water via neutralization, separation of precipitates, and waste water treatment was not easy because a large scale of facility, much labor, and the like were needed. In addition, since sulfate ion remains during extractant, there is a risk that mother liquid for extraction is contaminated with sulfate ion when this extractant is used for extraction again. If sulfate ion is mixed in a chloride type extraction liquid, electrode is deteriorated in the subsequent electrowinning step and raise voltage of electrolytic bath, resulting in cost increase. Furthermore, the extractant after removing the impurity elements by the above method has to be subject to the treatment to add chlorine by contacting with a solution containing chloride such as hydrochloric acid. This process required much cost and labor.
Under such circumstance, a more efficient scrubbing method is demanded, by which it is achieved that formation of precipitates and crud is not accompanied when extracting capability of an amine type extractant supporting a chloro complex ion of metal such as iron, zinc is regenerated, and that no treatment to add chlorine to the extractant after removing the impurity elements is required and operational material cost is low, and the like.
[Patent Literature 1]: JP-B-3-40089 (page 1)
[Patent Literature 2]: JP No. 3844752 (pages 1 and 2)